Method and apparatus for producing carbon dioxide pellets

ABSTRACT

There is provided a method and apparatus for practicing carbon dioxide particle blasting utilizing dense carbon dioxide pellets which have been formed by converting the solid carbon dioxide into liquid, flowing that liquid within a plurality of die openings, and transforming the liquid back into the solid phase while still within the die opening. This method includes the formation of a carbon dioxide cake within a snow barrel, urging the carbon dioxide cake against an upstream surface of a die plate having a plurality of die openings with such pressure such that the carbon dioxide becomes liquified within or adjacent the entrance of the die opening due to the pressure, responding as a super viscous liquid, and flowing within a portion of the respective die openings as a liquid. Along the length of the die openings, the liquid changes phase back to the solid state, while the die openings maintain compression (accompanying the back pressure) on the CO 2  to create pellets having a high density. The pellets are thereafter transported to an application nozzle and directed against a workpiece.

TECHNICAL FIELD

The present invention relates generally to particle blasting apparatusesand methods, and is particularly directed to apparatuses and methods formanufacturing sublimable pellets for use in cryogenic particle blasting.The invention will be specifically disclosed in connection withapparatuses and methods for producing carbon dioxide pellets and forusing the carbon dioxide pellets in a carbon dioxide blasting system.

BACKGROUND OF THE INVENTION

Cryogenic particle blasting is now relatively well known. The processgenerally utilizes sublimable particles, such as carbon dioxide pellets,which are propelled against a work piece for a variety of reasons, suchas, for example, to remove contaminates or coatings from the surface ofthe work piece Although carbon dioxide is referred to herein, it will beunderstood that any cryogenic sublimable material may be used.

The effectiveness and efficiency of the cryogenic blasting processdepends at least in part upon the type of contaminate or coating beingremoved and the nature of the surface from which it is being removed.Problems which typically arise in utilizing CO₂ particle blasting toremove contaminates or coatings from a surface include attainingcomplete coating or contaminate removal from the surface, attaining anacceptable removal rate, and preventing damage to the underlying surfaceor substrate. This is particularly a problem when carbon dioxideblasting is used to remove surface coatings from aircrafts wherein thesubstrate thickness is as low as 0.020 inches. In such an application,the carbon dioxide blasting process must be sufficient to remove thesurface coating without damaging the thin substrate by creating stressbuildup therein or work hardening the surface.

Several prior art methods and apparatuses are known for CO₂ particleblasting. Some of these are set forth in U.S. Pat. Nos. 4,947,592,5,018,667 and 5,050,805, and co-pending U.S. patent application07/781,326 filed on Oct. 22, 1991, now U.S. Pat. No. 5,188,151, all ofwhich are incorporated herein by reference. The process typicallyincludes the formation of carbon dioxide pellets by producing carbondioxide snow which is formed into carbon dioxide pellets by forcing thesnow through circular die openings. As disclosed by U.S. Pat. Nos.4,947,592 and 5,018,667, one method of forcing the CO₂ snow through dieopenings is by use of a piston. Other apparatuses for forming CO₂pellets include rotary pelletizers.

The diameters of the prior art pellets, as dictated by the diameter ofthe die opening, are 0.120 inches and larger. After producing thepellets, they are then transported by a transport gas to an applicationnozzle designed to accelerate the transport gas flow and the entrainedpellets to a high velocity. This exiting flow is directed at the workpiece.

While the prior art pellets produced an acceptable removal of coatingsor contaminates on a substrate, the pellets' size and speed createddeleterious effects on the substrate itself. A cryogenic flow of pelletssufficient to produce complete coating or contamination removal at anacceptable rate can damage the substrate by deforming it and creatingstresses.

Thus, there is a need in the art for a method and apparatus whichprovides sufficient coating or contaminate removal at an acceptableremoval rate, which does not damage the substrate.

SUMMARY OF THE INVENTION

Accordingly, it is a primary object of the present invention to providea method and apparatus for effectively and efficiently removing acoating or contaminate from a substrate without damaging the substrate.

It is another object of the present invention to provide a method andapparatus for producing pellets of a sublimable material which can beused for the effective and efficient removal of coatings andcontaminates from a substrate in a particle blasting process withoutdamaging the substrate.

It is yet another object of the present invention to provide a methodand apparatus for producing pellets made of a sublimable cryogenicmaterial which are smaller than those previously known to be used in theprior art.

Yet another object of the present invention is to provide a method andapparatus for producing cryogenic particles having a high weightdensity, which in turn results in the particles possessing a greaterapparent surface hardness.

Additional objects, advantages and other novel features of the inventionwill be set forth in part in the description that follows and in partwill become apparent to those skilled in the art upon examination of thefollowing or may be learned with the practice of the invention. Theobjects and advantages of the invention may be realized and obtained bymeans of the instrumentalities and combinations particularly pointed outin the appended claims.

To achieve the foregoing and other objects, and in accordance with thepurposes of the present invention as described herein, there is provideda method and apparatus comprising the practice of carbon dioxideparticle blasting utilizing carbon dioxide pellets having a diameter ofless than 0.120 inches, and increasing the flux density of pelletsstriking the workpiece.

In accordance with another aspect of the present invention, there isprovided a method and apparatus for practicing carbon dioxide particleblasting utilizing dense carbon dioxide pellets which have been formedby converting the solid carbon dioxide into liquid, flowing that liquidwithin a plurality of die openings, and transforming the liquid backinto the solid phase while still within the die opening. This methodincludes the formation of a carbon dioxide cake within a snow barrel,urging the carbon dioxide cake against an upstream surface of a dieplate having a plurality of die openings with such pressure such thatthe carbon dioxide becomes liquified within or adjacent the entrance ofthe die openings due to the pressure, responding as a super viscousliquid, and flowing within a portion of the respective die openings as aliquid. Along the length of the die openings, the liquid changes phaseback to the solid state, while the die openings maintain compression onthe carbon dioxide to create pellets having a high density. The pelletsare thereafter transported to an application nozzle and directed againsta workpiece.

Still other objects of the present invention will become apparent tothose skilled in this art from the following description wherein thereis shown and described a preferred embodiment of this invention, simplyby way of illustration, of one of the best modes contemplated forcarrying out the invention. As will be realized, the invention iscapable of other different embodiments, and its several details arecapable of modification in various, obvious aspects all withoutdeparting from the invention. Accordingly, the drawings and descriptionswill be regarded as illustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings incorporated in and forming a part of thespecification illustrate several aspects of the present invention, andtogether with the description serve to explain the principles of theinvention. In the drawings:

FIG. 1 is a fragmentary, side elevational view showing the pelletizer,hopper, feeder and air block, constructed according to the presentinvention.

FIG. 2 is an enlarged, fragmentary cross-sectional view of a pelletizerconstructed according to the present invention.

FIG. 3 is an end view of one embodiment of the die plate according tothe present invention, including an enlarged, fragmentary view thereof.

FIG. 3A is an end view of one embodiment of the die plate according tothe present invention, including an enlarged, fragmentary view thereof.

FIGS. 3B, 3C, and 3D are enlarged, fragmentary cross-sectional views ofdie openings constructed in accordance with the teachings of the presentinvention.

FIG. 4 is an end view of the die backing block for use with the dieplate illustrated in FIG. 3.

FIG. 4A is an end view of the die backing block for use with the dieplate illustrated in FIG. 3a.

FIG. 4B is an exploded, perspective view of the die backing block ofFIG. 4A.

FIG. 5 is an end view of the pelletizer showing the die backing block ofFIG. 4, taken along line 5--5 of FIG. 2.

FIG. 6 is an enlarged, fragmentary, partial cross-sectional view of thedischarge end of the pelletizer and the cutter assembly.

FIG. 7 is a cross-sectional view of the pellet cutter assembly.

FIG. 8 is an end view of the cutter knife assembly.

FIG. 9 is an enlarged view of the hopper assembly, partially cut away toshow the auger assembly within the hopper.

FIGS. 10, 11 and 12 are front, side and end views, respectively, of theauger assembly.

Reference will now be made in detail to the present preferred embodimentof the invention, an example of which is illustrated in the accompanyingdrawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

According to one of the methods of the present invention, the particleblast cleaning process, and in particular, the carbon dioxide particleblast cleaning process, is carried out utilizing generally cylindricalCO₂ pellets having a diameter of less than 0.120 inches. In this newmethod, the CO₂ pellets are formed in a pelletizer having die openingswhich are configured to produce pellets having a diameter less than0.120. The lengths of the pellets produced are substantially uniform,typically having a length to diameter ratio of 2.5:1, but ranging as lowas 1:1.

According to the various embodiments of this method of the presentinvention, pellets having diameters of 0.080 inches, 0.060 inches and0.040 inches may be utilized in practicing this method of the invention.Contrary to the prior art, which has focused on maximizing the size ofthe pellets while maintaining the flowability of the pellets through theprocess, the embodiments of this mathod of the present invention aredirected to maximizing the flux density of the pellets striking thesurface. The flux density is the number of pellet impacts per unit timeper unit area. At the same mass flow rate of CO₂, the number ofindividual pellets striking the work surface is greater with the smallerpellets, and this increased flux density is more efficient at removingcoatings or contaminates than the Prior art processes using the largerpellets and corresponding lower flux densities. The increasedeffectiveness of the higher flux density attained by using these smallerpellets allows the operator to reduce the Pressure of the transport gas,thereby lowering the individual Pellet velocity resulting in lesskinetic energy being imparted to the surface, thereby reducing orpreventing damage to the work piece.

By utilizing this new method incorporating the smaller pellets, greatereffectiveness and higher rates of coating or contaminate removal may beobtained. It is believed that the smaller pellets are more evenlydistributed throughout the discharge flow of the exit nozzle of theblasting system, and can be accelerated to a higher velocity at the exitof the nozzle by a given flow of transport gas than the larger prior artpellets could be.

Another method of the present invention is directed toward the processby which the CO₂ pellets are formed. The prior art processes for formingCO₂ pellets utilize mechanical shear to extrude solid carbon dioxidethrough die openings. This shear extrusion process limits the density ofthe pellets which can be created. The shear is accomplished by forcingthe CO₂ snow through the die opening while in the solid state. Althoughsome liquification of the CO₂ has been noted to occur adjacent theinterior walls of the die openings, such liquification has been at lowliquid pressures and de minimus, with the primary process beingmechanical shearing.

However, according to this method of the present invention, the carbondioxide pellets are formed by plasticizing the solid CO₂ allowing it toflow through the die openings. Plasticizing the solid CO₂ creates asuper viscous liquid CO₂ (such as occurs under high pressure such as5,000 psi) which flows as a fluid into the die opening, rather thanbeing extruded as a sheared solid through the die opening. Thus,substantially all of the CO₂ immediately downstream of the entrance ofthe die opening is a liquid, and more particularly, acts as a superviscous liquid.

In this process, as the CO₂ travels the length of the die opening, itchanges to the solid phase within the die opening once the pressure ofthe liquid drops low enough along the length of the die opening to allowthe liquid to change phase to the solid state. The pressure on the CO₂at the die opening exit is approximately atmospheric, so the pressuredrop across the die plate (i.e. along the length of the die opening)depends upon the pressure at the entrance to the die opening.

The creation of CO₂ pellets by liquifying the CO₂ solid cake at theentrance of each die opening under high pressure and causing the liquidto flow through the die opening and transform back into solid within thelength of the die opening can be used to make various sizes of CO₂pellets, including pellets having diameters greater than 0.120. It isparticularly applicable to pellets having diameters less than 0.120. Aswill be described below in connection with an apparatus for practicingthis method, the super viscous liquid CO₂ flow within the die openingsis created by subjecting the solid CO₂ cake located upstream of andadjacent the die plate within the pelletizer to pressures ofapproximately 5,000 psi. The practice of this method results fromcoordination and control of the pelletizer piston speed and theconfiguration of the die openings. The shape, size, length and number ofdie openings must enable the super viscous liquid to be formed at thedie opening entrance, and to change phase to solid along the length ofthe die opening. For example, if the piston "overdrives" the die plate(i.e., e.g. at high piston speeds) the liquid to solid transformationfront will move closer to the exit of the die opening, producing CO₂snow rather than dense pellets. "Underdriving" the piston will increasethe length of time required to make the pellets, but will still producethe dense pellets through plasticizing.

In practicing this method of the present invention, it is desirable topromote laminar flow of the super viscous liquid within the die opening.To do so, the entrance of the die opening is configured to reduceturbulence and preferably to result in a Reynolds number under 2000.

By practicing this method of the present invention, it is believed to bepossible to produce "super" dense pellets, approaching and exceeding thetheoretical maximum density of carbon dioxide of 97.6 lb/ft³ at 1Atmosphere. As an example of the super density attainable by utilizingthis super viscous liquid fluid flow process, the inventors have beenable to create long "spaghetti" like continuous strings of solid carbondioxide which can be tied in a knot. It is possible the super density ofsuch CO₂ pellets may be limited in time as the solid CO₂ has nocrystalline structure which can retain the internal stresses. Thus, itis believed, that over time the density of many of the super densepellets may decrease. In the CO₂ particle blast process, the pelletsflow through the system so fast that the pellets may reach theapplication nozzle before such reduction of density.

Because the solid CO₂ formed according to this method tends to retainits "spaghetti" like shape downstream of the die plate, a cutter isprovided adjacent the downstream side of the die backing block toproduce pellets of substantially uniform lengths. The specific cutterwill be described below.

Referring now to the figures, there is shown an embodiment of anapparatus suitable for practicing the various methods of the presentinvention. FIG. 1 is a side fragmentary elevational view showingpelletizer assembly 2, with elbow 4 connected thereto, and disposed toreceive pellets formed by pellet assembly 2. Transition assembly 6 isinterposed between elbow 4 and hopper assembly 8, configured to directCO₂ pellets into hopper assembly 8 from elbow 4. Tube 10 connects exit12 of hopper assembly 8 with receiving station 14 of feeder assembly 16.Air block 18 is located adjacent discharge station 20, whereat CO₂pellets are introduced into the transport gas flow. Discharge hose 22 isconnected to outlet 24 of air block 18, and receives the transport gaswith the entrained CO₂ pellets, and carries them to the applicationnozzle (not shown).

Referring also to FIG. 2, there is shown an enlarged, fragmentary,cross-sectional view of pelletizer assembly 2 and hydraulic system 26.Pelletizer assembly 2 includes snow barrel 28 which defines internalsnow chamber 30 into which CO₂ snow is injected by a phase changeinjection nozzle (not shown) similar in function to that described inU.S. Pat. No. 5,018,667, and located in ports 32. Liquid CO₂ isdelivered to the injection nozzles through CO₂ line 34. Although fourports and CO₂ lines are illustrated, more or less injectors may be used.As the liquid CO₂ is transformed to CO₂ snow by the phase changeinjection nozzles, the CO₂ is deposited in chamber 30. During this phasechange process, some of the CO₂ liquid will be introduced into chamber30 as gas. Depending upon the efficiency of the phase change injectionnozzles, as much as 50% of the CO₂ liquid may be flashed to gas in thisprocess. Barrel 28 includes a plurality of openings 36 which allow theCO₂ gas to be vented from chamber 30. A collector (not shown) is locatedadjacent openings 36 to collect the CO₂ gas, directing it through a heatexchanger (not shown) disposed to cool the incoming the CO₂ liquid inCO₂ lines 34. The collector also includes very fine screens (not shown)which are located adjacent openings 36, thereby preventing the CO₂ snowfrom flowing out through openings 36, while still allowing the CO₂ vaporto escape therethrough. At this point in the process, the internalpressure of chamber 30 is approximately atmospheric, so the CO₂ snow isnot forced out through the screens.

Connected to snow barrel 28 is hydraulic system 26 which includes ram 38disposed to be extended into chamber 30 and retracted therefromhydraulically. Connected to ram 38 is piston 40 which is disposed ininternal chamber 30. Piston 40 has about 0.100 inches of clearance oneach side between it and internal wall 42 of barrel 28. Wear rings 44are made of plastic and disposed in annular grooves formed about piston40. Wear rings 44 engage internal wall 42 about their outside diameter,thereby sealing chamber 30 and substantially preventing CO₂ snow fromleaking past both wear rings 44 as piston 40 is advanced toward dieplate 46. Wear rings 44 also function to maintain the alignment andspacing of piston 40 relative to internal wall 42. In this embodiment,piston face 40a includes raised portion 40b, which was found to producemore even compression of the CO₂ cake across the area of die plate 46.

Typically, the cycle time for pelletizer is 24 seconds of producing CO₂snow followed by 4-5 seconds of compression by piston 40, and 7-8seconds of retraction.

Die plate 46 is disposed at exit end 48 of pelletizer assembly 2, and isretained in a stepped bore formed in end cap 50 by die backing block 52which is connected by fasteners 54 to end cap 50. As illustrated by FIG.2, elongated fastening rods 56 retain end cap 50 to pelletizer assembly2.

The inside diameter of internal wall 42 of barrel 28 is illustrated asincluding step 58 leading to a larger internal diameter which is alignedwith and the same size as internal diameter 60 of end cap 50. Disposedwithin this recessed area is annular ring 62 which functions as athermal barrier. Annular ring 62 is made of plastic. This thermalbarrier helps insulate the CO₂ cake from the walls of snow barrel 28,since the CO₂ cake remains at this location during "dead" times, therebymaintaining the integrity of the CO₂ cake. The pelletizer efficiency isbased on the pressure drop across the die plate and the temperature ofthe CO₂ at the inlet to the die plate relative to the temperature at theoutlet. Typically, after running for 10 to 15 minutes, the temperatureof die plate 46 is stabilized at its minimum temperature, therebyproviding efficiency.

Referring now to FIG. 3, there is shown an end view of one embodiment ofdie plate 46, as it would appear removed from pelletizer assembly 2. Dieplate 46 includes a plurality of die openings 64 arranged in a pluralityof clusters 66. The typical arrangement of die openings 64 is shown inthe enlargement of a cluster 66 in FIG. 3. However, for clusters 67, thedie opening arrangement is rotated 90° from that shown in theenlargement of FIG. 3. FIG. 4 illustrates the embodiment of die backingblock 52 corresponding to die plate 56 of FIG. 3. As shown in FIG. 2,backing block 52 has a substantial thickness relative to die plate 46,and provides the support to maintain the position and dimensional shapeof die plate 46 against the pressure of the CO₂ cake (not shown) formedin chamber 30 adjacent die plate 46 by extending piston 40 to its fullextended position, which is approximately 25 mm from die plate 46. Aspreviously mentioned, according to the teachings of the presentinvention, the pressure at the interface between the CO₂ cake (notshown) and the upstream surface 68 of die plate 46, and moreparticularly at the entrance of each die opening, during extension ofpiston 40 typically reaches pressures of 4500 psi and up to 5,000 psi,which is limited by the currently available hydraulic rams and pistons.Higher pressures, when achievable, may be used to practice theinvention. Backing block 52 includes a plurality of exit openings 70which, when located adjacent die plate 46 in pelletizer assembly 2, arealigned with clusters 66. FIG. 5 illustrates the external end view ofexit end 48 of pelletizer assembly 2 with backing plate 42 mounted toend cap 50 by fasteners 54.

Referring again to FIG. 3, dashed line 72 indicates the inside diameterof annular ring 62 which closely conforms to the inside diameter ofchamber 30, and concomitantly the outside diameter of the CO₂ cakeformed by compression of the CO₂ snow. It is noted that in thisembodiment of die plate 46, there are regions 74 of die plate 46 createdby the arrangement of clusters 66 which fall within the outside diameterof the CO₂ cake. Regions 74, as do lands 76 between clusters 66 andlands 78 between die openings 64, "dead head" against the CO₂ cake.Lands 76 and 78, in conjunction with the shape of die openings 64beneficially contribute to creating the pressure necessary to plasticizethe CO₂ cake at the entrance to the die openings so as to produce thesuper viscous liquid within the entrance of the die opening 64 whichflows from the entrance substantially all as a liquid, in accordancewith the present invention. However, regions 74 are of such a size andlocation that the dead heading of the CO₂ cake thereagainst can resultin deleterious effects on the process of forming the CO₂ pellets throughdie openings 64 located adjacent regions 74. This configurationgenerally causes the CO₂ about the outside diameter of the CO₂ cake toflow radially inward and into die opening 64 as schematicallyillustrated by flow arrow 80 of FIG. 2. This represents an unbalanceddie plate, producing irregular flow at the edges. This flowcharacteristic presented some problems for die plate 46 when thediameter of die opening 64 was approximately 0.080 inches, although itdid not prevent the practice of the present invention with die plate 46.However, when smaller die openings were used, such as 0.060 inches and0.040 inches, this dead heading caused substantial deleterious effectson the process of forming to the CO₂ pellets. Therefore, anotherembodiment of the die plate, identified as die plate 46a in FIG. 3A canused. Die openings 64a are arranged in groups 66a in a tightly packedpattern as shown in the enlargement of die opening 64a in FIG. 3a. Thispattern represents a substantial reduction in the lands 78a between dieopenings 64a as well as lands 76a between groups 66a. This arrangementof die openings 64a minimizes dead head region 74a about the outside ofthe pattern of die openings 64a, thereby minimizing or substantiallyeliminating the deleterious effects of the outer peripheral regions ofthe CO₂ cake. By way of example, for die openings 64a having a nominalinternal diameter of 0.060 inches, the center to center horizontalspacing (as shown in FIG. 3A) was nominally 0.080 inches, and thevertical distance between the centers of die opening 64a was nominally0.080 inches. For die openings 64a having a nominal diameter of 0.040inches, these center spacings were nominally 0.060 inches.

FIGS. 4A and 4B illustrate die backing plate 52a which corresponds todie plate 56a. As shown most clearly in the perspective view of FIG. 4B,die backing plate 52a includes annular support 52b which has a pluralityof slots 52c for receiving and retaining a plurality of respectivesupport plates 52d and 52e which align with lands 76a of die plate 46a.

Referring now to FIGS. 3B and 3C, there are shown cross sections ofvarious profiles of die openings 82 and 82a which may be used as dieopenings 64 or 64a. In forming the die openings, there is a balancebetween the cost of manufacture of the die plate and the efficientdesign of the die opening. In terms of the manufacturing process, themost cost effective manner to form such openings through the die platewould be to stamp the openings. However, the diameter of the openings,the number of the openings, as well as the smallness of the landsbetween the openings make stamping a non-viable method, particularly forsmaller openings, such as 0.060 inches and 0.040 inches. Drilling theholes is an equally unattractive manufacturing method, both because ofthe number of holes and expense associated therewith. Other methods ofmanufacturing include electrical discharge machining (EDM) and laser.Both of these methods are expensive, although the laser machining of thesmall (0.060 inches and 0.040 inches) die openings is less expensivethan EDM. The die plate is made of stainless steel, both because of itscorrosion resistance and because of the nature of its machinability. Theinternal bores of the die openings formed in the die plate have aroughness, which is probably in the range of 64-125 micro inches. Thisroughness contributes to maintaining compression in the die opening andmaintaining back pressure. Smooth die open bores do not work as well.

Keeping in mind that the configuration of the die opening and theentrance is selected to promote non-turbulent, laminar flow having aReynolds number lower than 2,000, in FIG. 3B, there is shown die opening82 having counterbore 84 formed adjacent upstream surface 86 of dieplate 88. The exact configuration of die opening 82 and counterbore 84has varied in practice in dependence upon the nominal inside diameter ofdie opening 82, and the method of manufacture. For example, with respectto a nominal diameter of 0.080 inches, wherein the die openings aredrilled through the die plate having a thickness of 0.090 inches,counter bore 84 had a depth of 0.015 inches and an included angle 90 of60°. Die opening 82 was tapered from the end of counter bore 84 towithin approximately 0.030 inches of downstream surface 92 with a #5/0taper pin reamer. As another example, die openings having a nominaldiameter of 0.060 inches and 0.040 inches are machined by laser beam.The laser beam process, in which die opening 82 is cut from upstreamsurface 86 toward downstream surface 92, produces an extremely preciseopening at downstream surface 92. As a result of the process, dieopening 82 tapers inwardly toward downstream surface 92 resulting in aslight decrease in diameter along the length of die opening 82 in thedownstream direction. In this process, counter bore 84 is formed to"clean up" the entrance to die opening 82 which was formed by theremoval of molten metal created by the laser beam process. For dieopenings having a nominal diameter of 0.060 inches, through a die platehaving a thickness of 0.090 inches, a counter bore having included angleof 82° was machined to a depth of 0.010 inches, and had a diameter atthat depth of 0.064 inches. For die openings having a nominal diameterof 0.040 inches, formed through a die plate having a thickness of 0.105inches, the counter bore had a depth of 0.010 inches and an includedangle of 60°. The diameter of the counter bore at this depth was as highas 0.046 inches.

Referring now to FIG. 3C, die opening 82a is shown having ellipticalentrance 84a adjacent upstream surface 86a of die plate 88a. Thiselliptical entrance is configured to promote laminar flow of the CO₂liquified therein and prevent turbulent flow, thereby improving thequality of the CO₂ pellet emanating from die opening 82a. By way ofexample, FIG. 3D illustrates the location of an elliptical entrance andthe formula for the radius of curvature for a die opening having anominal inside diameter of 0.045 inches and a die plate thickness of0.125 inches. Other smooth transitional shapes may also be used whichpromote laminar flow.

There are several different physical attributes of die openings 82 whichmay be considered. The open area ratio of the die plate, which is thetotal open area of all of the die openings divided by the entire area ofthe piston face is related to controlling the back pressure at theupstream face of the die plate, based on well known flow equations forfluids. Another physical characteristic is the total wetted perimeter,i.e. the sum of the basic hole circumferences of all die openings in thedie plate. This can be used to determine a total wetted perimeter toopen area ratio. The higher this ratio, the more pressure required topush the CO₂ through the die openings. The total wetted perimeter isrelated to the amount of friction opposing the flow of the CO₂ throughthe die openings. This resistance to flow, together with the highviscosity of the plasticized CO₂ combine to result in the back pressurewhich exists at the interface between the CO₂ cake and the upstreamsurface of the die plate. Empirically, it has been noted that a totalwetted perimeter to total open area ratio of about 50/inch produces highquality, dense pellets. Higher ratios create greater resistance to flow,although ratios as high as 66/inch have resulted in acceptable, goodpellets. Too high of a ratio can stall the piston.

By way of example, for die openings having nominal diameters of 0.080inches, arranged as illustrated in FIG. 3, 912 die openings were usedfor a total actual open area of 4.584 in². When combined with a 6 inchround piston, which has an area of 28.274 in², the open area ratio is0.162. For the 912 die openings, the total wetted perimeter is 229.210inches, producing a wetted perimeter to total open area ratio of50.002/in..

As another example, for die openings having nominal diameters of 0.060inches, and arranged as illustrated in FIG. 3A, 3,128 die openings wereused for a total actual open area of 8.844 in². The open area ratio was0.313. The total wetted perimeter was 589.614 inches, resulting in awetted perimeter to total open area ratio of 66.668/in.

For best results, it has been found that the aspect ratio of the dieopening, i.e., the thickness of the die plate to the diameter of the dieopening, must be at least 1.0. It has also been found that an aspectratio of 3.0 can result in stalling the hydraulic system, which exertsforces up to 5,000 psi at the interface between the upstream surface ofthe die plate and the CO₂ cake, and more particularly within theentrance to the die opening.

Referring now to FIG. 6, which is a fragmentary enlarged partialcross-sectional view of exit end 48 having elbow 4 connected thereto soas to receive CO₂ pellets discharged by pelletizer assembly 2, there isshown cutter assembly 94 disposed therewithin. Cutter drive motor 96 iscarried by elbow 4, and has drive shaft 98 which extends therethrough toengage cutter assembly 94. Upper diverter 100 is shown in FIG. 6, andcomprises actuator cylinder 102 supported at one end by motor 96, andconnected at the other end to diverter door 104. During the initialchill down cycle of the particle blast cleaning apparatus, diverter door104 is left open by actuation of cylinder 102 during the first 9 strokesof piston 40. This is because during chill down, good quality pelletsare not produced by pelletizer assembly 2, and are diverted outside ofthe equipment. Diverter door 104 is automatically closed by the controlsystem for the particle blast cleaning apparatus after 9 strokes ofpiston 40 have occurred. Nine strokes was determined empirically.

Referring also to FIGS. 7 and 8, cutter assembly 94 is illustratedhaving a plurality of cutting blades 106 extending radially outward fromshaft 108 to support ring 110. Interposed between cutting blades 106 andextending radially outward from shaft 108 are additional cutting blades112. The distal ends of cutting blades 112 may alternatively besupported by a cross member (not shown) extending between respectivepairs of blades 106. Such a cross member may also act as a cuttingblade. The inner ends of blades 106 and 112 are welded together, andshaft 108 is welded thereto.

Cutting blades 106 and 112 are disposed adjacent downstream surface 114of die backing block 52. By rotation of cutter assembly 94, thecontinuous individual pieces of solid CO₂ discharging from eachindividual die opening 64 is cut by blades 106 and 112 intosubstantially uniform lengths.

Cutter assembly 94 includes stainless steel hub 116 which receives driveshaft 98. Hub 116 is keyed to drive shaft 94 by key 118 which transmitsthe torque from drive shaft 98 to cutter assembly 94. A plurality ofarched drive members 120 are welded to hub 116 at locations 122. Therespective distal ends of drive members 120 are welded to respectivecutting blades at locations 124. Disposed about hub 116 and a portion ofshaft 108 is insulating member 126 which includes a frustoconicallyshaped portion 126a which blends into a conical portion 126b whichextends about stainless steel hub 116. Set screw 128 extends throughinsulating member 126, hub 116 and into a hole drilled in drive shaft 98so as to locate cutter assembly 94 axially on drive shaft 98. The distalend of shaft 108, which includes hub 109 is supported by member 126, andis inserted through member 126 as shown before shaft 108 is welded toblades 106 and 112 at its the proximal end.

Insulating member 126 is machined from UHMW plastic and functions as athermal barrier to reduce the transfer of heat from motor 96 to cutterassembly 94.

Frustoconically shaped portion 126 is so shaped to promote the smoothflow of pellets being discharged by pelletizer 2. By way of example, itis noted that during the full stroke of piston 40 (1/2 inch to 1/4 inchper second), for 0.080 inch diameter die openings, each strand of solidCO₂ can advance about 25" having a speed of 7-9 inches per second.Typically, on the average, 6.5 pounds of pellets are produced by eachstroke of piston 40.

The rotational speed of cutter assembly 94 is selected so as to providethe desired length of CO₂ pellets. The range of speed utilized thus farhas been between 50 RPM-500 RPM. This speed may be adjusted duringoperating of the equipment so that the length of the CO₂ pellet can becontrolled in process. Alternatively, rather than being separately andmanually adjustable, the speed could be set as part of the processcontrol for all of the equipment.

Referring now to FIG. 9, hopper assembly 8 is illustrated, with hopper 9partially cut away to show rotating auger assembly 130. The CO₂ pelletsflow through transition assembly 6 into hopper 9. As disclosed in U.S.Pat. No. 4,947,592, in order to avoid agglomeration of CO₂ pellets inhopper 9 and to advance pellets into receiving station 14 of feeder 16,auger assembly 130 extends through the interior cavity of hopper 9,through exit 12 such that end 132 of auger assembly 130 is located,almost in line to line contact with the rotor (not shown) of feeder 6 atreceiving station 14. Hopper assembly 8 includes lower diverter assembly134 which may be opened to dump CO₂ pellets out of the hopper to theenvironment. Such discharge is utilized to empty hopper 9 when theequipment will not be used for a period of time so as to avoidagglomeration of the pellets. Such discharge may occur as part of theprocess control for the equipment to occur automatically upon apredetermined time lapse of non operation, for example after 15 minutesof non use.

Referring also to FIGS. 10, 11 and 12, auger assembly 130 includes acontinuous inclined surface 136 extendinq from auger shaft 138. Augershaft 138 is rotated by any suitable means, such as motor 140 mountedatop hopper assembly 8. A plurality of agitation rods 142, 144, 146 and148 extend from shaft 138. Agitation rods 142 and 144 are shapedgenerally complimentary to the section of hopper 9 adjacent which theyare disposed, being generally inclined inwardly and downwardly. It isnoted that there is approximately 1/8th inch clearance between agitationrods 142, 144, 146 and 148 and hopper 9. If agitation rods 142, 144, 146and 148 are too close to the internal sides of hopper 9, the CO₂ pelletscould be ground by the agitation rods causing them to pack tightly andagglomerate. Agitation rods 146 and 148 are also shaped complimentary tothe corresponding section of hopper 9 and tube 10 adjacent which theyare located, being generally vertical. Circular support 150 extendsbetween agitation rods 146 and 148.

Connected to distal end 152 of auger shaft 138 is ring assembly 154,including annular ring 156 which defines end 132 of auger assembly 130and is supported by auger shaft 138 by support members 158. Thisconfiguration allows pellets to flow through the interior of annularring 156 and directly into receiving station 14 of feeder 16.

The rotor (not shown) of feeder 16 may be a single cavity rotor, a dualcavity rotor, or even a dual rotor as set forth in U.S. Pat. No.4,947,592.

The foregoing description of a preferred embodiment of the invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed. Obvious modifications or variations are possible in light ofthe above teachings. The embodiment was chosen and described in order tobest illustrate the principles of the invention and its practicalapplication to thereby enable one of ordinary skill in the art to bestutilize the invention in various embodiments and with variousmodifications as are suited to the particular use contemplated. It isintended that the scope of the invention be defined by the claimsappended hereto.

We claim:
 1. A method for producing carbon dioxide pellets, comprisingthe steps of:(a) providing a die plate having a plurality of respectivedie openings formed therethrough, said die plate having upstream anddownstream surfaces, each said respective die opening having an entranceadjacent said upstream surface; (b) providing carbon dioxide in thesolid phase adjacent said die plate upstream surface; (c) urging saidcarbon dioxide in the solid phase toward said die plate with forcesufficient to cause said carbon dioxide to under go a first phase changefrom the solid phase to the liquid phase and flow within the saidentrance, thereby forcing said carbon dioxide to flow through saidrespective die openings such that substantially all of said carbondioxide flowing within a portion of said respective die openings is inthe liquid phase; and (d) flowing said carbon dioxide through saidrespective die openings at a rate such that said carbon dioxide undergoes a second phase change from the liquid phase to the solid phasewithin said respective die openings.